Semiconductor lasers of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such lasers can be used for amplifying optical signals immediately by optical means. This allows to design all-optical fiber communication systems, avoiding any complicated conversion of the signals to be transmitted which improves speed as well as reliability within such systems.
In one kind of optical fiber communication systems, the lasers are used for pumping erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance are 980 nm lasers with a power output of 100 mW or more, which wavelength matches the 980 nm erbium absorption line and thus achieves a low-noise amplification. InGaAs lasers have been found to serve this purpose well and are used today in significant numbers. However, the invention is in no way limited to InGaAs lasers.
There are examples of lasers of other wavelengths and materials for which the present invention is applicable. Generally, laser diode pump sources used in fiber amplifier applications are working in single transverse mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (FP) lasers. Three main types are typically used for erbium amplifiers, corresponding to the absorption wavelengths of erbium: InGaAsP and multiquantum-well InGaAs lasers are used at 1480 nm; strained quantum-well InGaAs lasers at 980 nm; and GaAIAs lasers at 820 nm.
One of the problems occurring when using semiconductor lasers for the above purpose is their wavelength and power output instability which, though small, still affects the amplification sufficiently to look for a solution to the problem. This problem is already addressed in U.S. Pat. No. 5,563,732 by Erdogan et al., entitled “Laser Pumping of Erbium Amplifier”, which describes the stabilization of a pump laser of the type described above by use of a Bragg grating in front of the laser. This grating forms an external cavity with the laser. The emission spectrum is stabilized by the reflection from the grating. The grating is formed inside the guided-mode region of the optical fiber at a certain distance from the laser. Such a fiber Bragg grating is a periodic (or aperiodic) structure of refractive index variations in or near the guided-mode portion of the optical fiber, which variations are reflecting light of a certain wavelength propagating along the fiber. The grating's peak-reflectivities and reflection bandwidths determine the amount of light reflected back into the laser.
Ventrudo et al. U.S. Pat. No. 5,715,263, entitled “Fibre-grating-stabilized Diode Laser” describes an essentially similar approach for providing a stabilized laser, showing a design by which the laser light is coupled to the fiber by focussing it through a fiber lens. Again, a fiber Bragg grating is provided in the fiber's guided mode portion, reflecting part of the incoming light back through the lens to the laser. To summarize, when positioning a fiber Bragg grating at a certain distance from the laser's front facet and when the laser gain peak is not too far from the Bragg grating's center wavelength, it is understood that the laser is forced to operate within the optical bandwidth of the grating and thus is wavelength-stabilized. Additionally, low-frequency power fluctuations seem to decrease by the effect of induced high-frequency multi-mode operation.
Though the above stabilization methods are effective, they all use active temperature stabilizing elements. None of the above prior art addresses solutions for high power (i.e. >100 mW) laser sources capable of stable operation without using an active temperature stabilizing element. Such cooling elements, commonly known as thermoelectric coolers (TEC), are usually attached to the heatsink of the laser for maintaining the laser temperature at a constant level. The need for TEC's contributes significantly to the complexity and cost of a laser source.
A so-called external cavity laser is known from from Bestwick et al. U.S. Pat. No. 6,101,210. This design, though mentioning that cooling of the laser may not ne necessary, is however limited to narrow bandwidth signal lasers whose power dissipation is much lower compared to the high power lasers addressed by the present invention. Also, Bestwick et al focus on production techniques and do not disclose any reflectivity values or ratios for the laser front facet and/or the grating. Thus, the low power, narrow bandwidth signal lasers disclosed by Bestwick et al. do not provide any indication of how to make or design an uncooled high power laser according to the present invention.
Thus, it is the main object of the invention to devise a simple and reliable laser source with sufficiently large locking range, especially for pump lasers in optical fiber communication systems, that provides a stable output without the need for an active temperature stabilizing element.
A specific object is to avoid the detrimental switching of the laser between operation at a single longitudinal mode and operation at multi-longitudinal modes, even for a laser output power of more than 100 mW, and thus increase the stability of the output of high power laser sources. Output stability shall be achieved for high optical power having reduced noise at low frequencies, wavelength stability and high side-band suppression outside the fiber Bragg grating bandwidth.
A further object is to allow maximum flexibility for choosing the laser's parameters without running into stability problems.
A still further object is to avoid any further complexity and keep the number of additional components of the laser source within a laser pumped optical amplifier to a minimum.